The present invention is related to receiver quieting and in particular to receivers having adjustable squelch control.
Squelch systems are commonly employed in VHF and UHF receivers to suppress noise when little or no signal is being received. More particularly, intelligence processing circuits, typically audio circuits, are turned on or off in accordance with excursions of a squelch drive signal across a threshold level. Generally, in order to change the threshold level, an adjustment internal to the receiver is required. The receiving equipment is generally remote and manual adjustment can be made only periodically.
Sometimes, however, users require provisions for an optional external squelch level control to permit the crew to adjust the squelch setting. Prior art approaches to providing such external control many times introduce one or more undesirable characteristics (see ARINC CHARACTERISTIC 566 by Aeronautical Radio, Inc., issued Oct. 17, 1968, Section 3.12.1). For instance, some receivers with adjustable squelch control are not interchangeable with receivers having only a preset squelch threshold. This is undesirable from both the manufacturers' and users' standpoint because it prevents standardization. Even interchangeale systems may require needless duplication of components, an expensive control system and/or unnecessarily complex interconnection and/or readjustment of internal squelch threshold. Both interchangeable and noninterchangeable systems sometimes provide external squelch adjustment only in one direction; i.e., either above or below the preset threshold. It can also happen that the adjustment is permanent in the sense that even after the extenal adjustment control is removed, the internal setting is no longer positioned for the preset level.
Thus it is an object of this invention to provide an improved receiver with an adjustable squelch control which overcomes the aforementioned difficulties.